It is important to know what level of CO your Honda Civic is producing before we can diagnose the high HC fault properly. HC is raw fuel (gasoline). High HC during the smog test means raw fuel is leaving your Honda Civic's tailpipe when it should be being burned in the combustion chamber.
Something is causing your Civic'c engine to not burn fuel as efficiently as possible. It is important to know your Honda's CO emission level is in order to diagnose this high HC failure. Since you mention you've already replaced the catalytic converter, spark plugs and checked the timing belt, we high recommend making your car is in proper fuel control; and not possibly running lean or rich
As mentioned, while diagnosing high HC faults it is important to know how much CO the vehicle produced during the smog check as well. The byproduct of incomplete combustion is CO (partially burned fuel).
High CO along with high HC indicates your Honda Civic might be running rich (more fuel then required entering the combustion chamber). If CO emissions are very low (0.00 to 0.05) and high HC, this is a possible indication of a lean fuel mixture causing a misfire due to an inadequate amount of fuel and subsequently increasing HC. In both scenarios, either high or low CO, HC emissions will be elevated.
If your Honda Civic failed for high HC but is producing normal CO emissions, we would suspect the vehicle's ignition system (spark) has a fault. If you are certain the ignition system is fine, go ahead and check that timing is correct by matching up the timing marks on the camshaft with the crankshaft; although if this was the case you should keep in mind HC mentions would probably be much higher than 100 ppm over the maximum limit plus your Honda Civic would feel sluggish during acceleration.
